The most convenient way to retrieve the raw print of an email is to connect thunderbird to Citadel via IMAP, search the message in question, click it, and press [ctrl] + [u]; Thunderbird will then open a popup window with the raw message. [File]-[save] will then allow you to save the message to 'message.eml' you can then gzip it, and post it as an attachment to the support forum.
In some situations, it is necessary to capture a message on the way in. You can do this with tcpdump capturing port 25 and use WireShark to extract the stream. You then can zip it, and post it to the support forum.